Steam methane reforming (SMR) processes are widely used in the industry to convert a feedstock (e.g., natural gas) into a syngas stream containing primarily hydrogen and carbon monoxide by undergoing a sequence of net endothermic reactions. Since the reforming reaction is endothermic, heat is supplied to the catalyst filled tubes located in the combustion section of the steam methane reformer by burners. Fuel for the burners comes from sources such as the pressure swing adsorption (PSA) unit in the form of purge gas, also known as tail gas. Some makeup fuel such as natural gas is also supplied to the burners. The following reactions take place inside the catalyst packed tubes:CnHm+nH2O<=>(n+0.5m)H2+nCOCO+H2O<=>CO2+H2 
The crude synthesis gas product (i.e., syngas) from the reformer, which contains mainly hydrogen, carbon monoxide, carbon dioxide, and water, is further processed in downstream unit operations, such as the shift reactor and the PSA unit. An example of steam methane reformer operation is disclosed in Drnevich et al (U.S. Pat. No. 7,037,485), and incorporated by reference in its entirety.
Typically, the syngas, which is produced from the reformer at high temperature, must be cooled to near ambient temperature for purification in the downstream PSA unit, which separates the stream into nearly pure hydrogen product and tail gas. It is this tail gas stream from the PSA which is recycled to the reformer burners. The products of the burner combustion (flue gas) are also cooled to recover their heat. These cooling needs are achieved in part by preheating process streams and steam generation.
The amount of heat to be recovered exceeds that needed to operate the process, and not all remaining heat can be effectively recovered via steam generation. This wasted heat reduces overall plant efficiency. One large stream that is often not heated is the PSA tail gas due to the high capital cost and pressure drop associated with conventional standalone tail gas preheaters. If this stream could be heated cost effectively without additional pressure drop, the overall efficiency of the process would increase due to a corresponding decrease in required makeup fuel.
Heating PSA tail gas has been disclosed in prior art. See, e.g., U.S. Patent Application Publication Nos.: 2009/0230359 A1, 2005/0178063 A1, 2006/0231463 A1, 2007/0051042 A1, 2009/0232729 A1, and U.S. Pat. No. 4,149,940. However, these standalone shell and tube or plate type heat exchangers are typically large and expensive. Only minimal pressure can be tolerated in the low pressure PSA tail gas stream before hydrogen recovery in the PSA is impacted, decreasing overall plant efficiency. In the related art, such as for example, U.S. Pat. No. 8,187,363 to Grover addresses the possibility of heating the tail gas stream through the use of a plate type exchanger prior to introducing same into the combustion zone of the SMR. Such an exchanger will still introduce undesired pressure drop and require significant capital cost. Therefore, it is desirable to heat the PSA tail gas stream with a minimal decrease in its pressure, while minimizing the additional capital required.
Regarding the PSA tail gas surge tank, the related art has focused on increased mixing within the surge tank vessel, as shown in U.S. Pat. Nos. 6,607,006 B2 and 6,719,007 B2. The focus of the '006 patent is “amplitude attenuation of time-variant properties of fluid stream,” which is the introduction of time variant streams into an enclosed volume to control the residence time distribution, with equations governing “flatness constraints” as shown in the document. The '007 patent also discusses “attenuating the amplitude”, with a focus on the mixing zone containing an inlet and three or more outlets for three or more individual fluid portions. There is no mention of heating within the surge tank vessel.
To overcome the disadvantages of the related art, it is one of the objectives of the present invention to provide a method for increasing the temperature of PSA tail gas by incorporating heat exchange surface within the PSA tail gas surge tank. This allows for heating of the PSA tail gas without an additional heat exchanger introducing additional pressure drop between the PSA tail gas surge tank and the SMR combustion zone.
It is another object of the invention to increase the overall plant efficiency of a hydrogen generation system through a novel means of increasing the temperature of the PSA tail gas. This invention adds the means to heat the PSA tail gas within the existing PSA tail gas surge tank(s).
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.